1. Field of the Invention
This invention generally relates to an optical image reading system for optically reading an original image for use in a facsimile machine or the like, and, in particular, to an image reading system using a light-emitting diode array as a light source.
2. Description of the Prior Art
An image reading system for reading an original image is used in various apparatuses, such as facsimile machines. In such an image reading system, a white level reference is typically provided for reflecting light from a light source, so that the light intensity level of the light reflecting from the white level reference is used as a reference to set a white level of the surface of the original to be read. In such an image reading system, use has been typically made of a fluorescent lamp tubular in shape as a light source. Since the fluorescent lamp is relatively large in size, its use tends to prohibit to make the entire image reading system compact in size. To cope with this problem, it has been proposed to use a light-emitting diode array as a light source; however, if use is made of a light-emitting diode of the divergent type as a light source in an image reading system, the light intensity level tends to be insufficient as compared with a light source, such as a fluorescent lamp. Accordingly, if use is made of a light-emitting diode as a light source of an image reading system, use must be made of a light-emitting diode of the non-divergent type or convergent type, which emits light substantially in parallel or in a convergent manner.
However, in the case of a light-emitting diode 1 of the convergent type, the light intensity distribution will have a shape as shown in FIG. 5, which rapidly changes in the widthwise direction. Thus, when such a convergent type light-emitting diode 1 is used as a light source of an image reading system for correcting the white level of the surface P.sub.2 of an original 3 using the light intensity level of the light reflected at the reflecting surface P.sub.1 of a white level reference 2, if the optical axis of the light-emitting diode 1 is aligned with the reflecting surface P.sub.1 of the white level reference 2 as shown in FIG. 6, the light intensity level is a maximum at the reflecting surface P.sub.1 of the white level reference 2; however, the light intensity level at the surface P.sub.2 of the original 3 becomes significantly reduced as shown in FIG. 5. As a result, a white level detected by a photoelectric element 4 will differ between the reflecting surface P.sub.1 of the white level reference and the surface P.sub.2 of the original 3, as shown in FIG. 7. Accordingly, if the original 3 has a half-tone image, the leading margin of the original 3 will be read as a black area, which is disadvantageous.
On the other hand, if the optical axis of the light-emitting diode 1 is aligned with the surface P.sub.2 of the original 3 as shown in FIG. 6, the light intensity level at the surface P.sub.2 of the original 3 is at maximum; however, the light intensity level at the reflecting surface P.sub.1 of the white level reference 2 is set at a significantly low level, as shown in FIG. 5. In this case also, a white level detected by the photoelectric element 4 will differ significantly between the reflecting surface P.sub.1 of the white level reference 2 and the surface P.sub.2 of the original 3, as shown in FIG. 8. As a result, if the original 3 has a half-tone image, the left and right margins of the original 3 will be read as black areas.